Design and evaluation of a new solar-biomass based energy system for a small sustainable residential community

In this paper, an integrated energy system is developed and analyzed, which generates power, heating and freshwater while being supplied with two distinct renewable energies: biomass and solar thermal. The system combines a multi-staged steam Rankine cycle with the steam generator energized via heat transfer from hot biomass combustion gases and hot solar thermal oil. It also includes a multi-effect desalination unit energized with thermal energy recovered from hot flue gases. The biomass combustion rate is controlled so that steam is generated steadily and at constant parameters regardless of the fluctuation and intermittence of solar radiation. At the same time, no thermal storage is required for solar energy. The steam Rankine cycle is optimized to steam extraction fraction so that the net power generation is maximized. The optimum steam extraction fraction of a low-pressure turbine is 5.5% of the main steam flow rate, which is the steam ahead of the high-pressure turbine. Due to solar radiation, biomass consumption is reduced to about half. Considering the solar radiation for a typical day in the Kingdom of Saudi Arabia, biomass consumption decreases three times during noontime (when solar radiation is the maximum) with respect to the nighttime. Nevertheless, the studied system generates more heating by-products and more freshwater when the biomass combustion rate is higher. During daytime, the current study achieves freshwater and heating production almost three times higher than the nightime requirement. The study results show that the need for typical solar thermal storage can be eliminated, and so the economic competitiveness of the solar thermal energy resource substantially increased. The maximum energy efficiency is slightly over 50% due to cogeneration, whereas the maximum exergy efficiency is 47%. The Rankine cycle destroys three times more exergy than the combustion chamber, whereas the other subsystems destroy significantly less exergy than the Rankine cycle.